A procedure in Prolog is a group of clauses about the same relation, for example:

is_a_parent(Parent) :- father_of(Parent, _Child).
is_a_parent(Parent) :- mother_of(Parent, _Child).

Here, two clauses together define the condition for someone to be a parent - together they form a procedure.

See also underscore for the reason for putting an underscore (_) character at the front of the variable _Child.